Cisco TDR Feature

I was troubleshooting an AP that was in another building. I thought it would come up with PoE but there seemed to be an issue. Further investigation lead to an interface status of up/down. A little weird for an AP.

Checking the configuration looked okay. After spending some time looking over the same information, I researched to see if there was any way to test an individual port. In the past, I’ve had to replace this specific line card because ports were going bad.

What is TDR

Cisco has a built in cable diagnostic feature called TDR, Time-Domain Reflectometer. TDR is used to test cables to find faults. There’s a good resource on the Cisco Support Forums.

Right away, the pair length wasn’t determined and the remote pair was invalid. Not what I want to see. By the way, the switch does a surprisingly great job and determining the cable length. I assumed I had a bad cable so I swapped that out. Unfortunately, I had the same results.

The stupid fix to this was powering the switch with a power adapter. Turns out the switch wasn’t giving PoE to the AP (another problem). After providing power to the AP, I re-ran the cable diagnostics and received the following output.

My Thoughts

TDR is a useful feature especially when troubleshooting remotely. I didn’t have a cable tester on-hand but the built-in Cisco TDR proved to be beneficial. My problem was not a bad cable although it seemed like it in the cable diagnostics results.